The present invention relates to a superconducting tape and in particular to a composite superconducting tape.
The invention has been developed primarily for use with high load current carrying cables and will be described hereinafter with reference to that application. It will be appreciated, however, that the invention is not limited to this particular field of use and is also applicable for use with coils, magnets, transformers, motors and generators as well as current carrying cables.
Tapes comprising superconducting material, and referred to as superconducting tapes, are already known. These tapes generally comprise one or many superconducting filaments in or, in some cases on the surface of, a medium of silver or silver alloy. One main class of superconducting tape is referred to as powder-in-tube or PiT tape. It is known to make this PiT tape by drawing or otherwise reducing a tube of silver or silver alloy which is filled with a powder form of the superconducting material. This tube is then usually further drawn, rolled or otherwise formed into a thin tape.
Multifilamentary tapes are mostly made by grouping filled tubes in a common silver or silver alloy sheath at an intermediate stage of reduction.
Composite powder-in-tube tapes are sometimes made by stacking individual tapes and securing them together. This securement is achieved in some instances by a wrapping of tape or in other instances the tapes are diffusion bonded and all the elements of the tape are longitudinal.
One important superconducting oxide is known as BSCCO-2223, and is a compound oxide of bismuth, strontium, calcium, and copper. It is known that certain limited substitutions can be made to this compound. It should also be noted that this compound that can be considered a cuprate salt.
When tapes are used to carry alternating current, rather than direct current, the superconducting tapes do not exhibit zero power loss. Notwithstanding, these are small compared to those exhibited by normal metallic conductors. This power loss resulting from the carrying alternating current rather than direct current is called xe2x80x9cAC lossxe2x80x9d. For an individual tape, the AC loss can be of the order of 100 microwatts per meter per ampere of critical current squared.
AC losses and various other properties of superconducting tapes are adversely affected by mechanical stresses, especially tensile stresses. This is a particular difficulty that arises from bending of the tapes such as that which occurs when the tapes are formed in to a cable. That is, the tapes are generally helically wound onto a former. In other applications, such as forming a coil, similar problems arise.
It is an object of the present invention, at least in the preferred embodiments, to overcome or substantially ameliorate one or more of the disadvantages of the prior art.
According to a first aspect of the invention there is provided a composite superconducting tape, characterised by at least one constituent superconducting tape including a pair of exposed opposite major faces and at least one outer layer metal tape overlying and bonded to one of the exposed major faces thereof with the proviso that if there are two such metal tapes bonded to the major faces they differ in a common strength property for selectively displacing the neutral flexural plane of the at least one constituent superconducting tape.
According to another aspect of the invention there is provided a composite superconducting tape including at least one constituent superconducting tape having two exposed opposite major faces, two opposite edges extending between the major faces and a flexural neutral plane extending substantially parallel to and intermediate the major faces, and an outer layer of metal tape bonded a first of the exposed major faces for selectively displacing the neutral flexural plane toward or away from the first major face.
Preferably, the composite tape is wound along a helical path onto a former such that the first metal tape includes a convex outer surface.
According to a second aspect of the invention there is provided a composite superconducting tape having a first major outer face and a second major outer face spaced from the first, characterised by at least one constituent superconducting tape and at least a first metal tape overlying and bonded to the first face whereby a bending of the composite superconducting tape such that the first major face is convex results in the at least one constituent superconducting tape being subject to a substantially compressive force.
Preferably, the composite superconducting tape includes a multiplicity of stacked and bonded constituent superconducting tapes. More preferably, the composite tape includes a second metal tape overlying and bonded to the second face.
Preferably also, in use, the composite tape is wound in the same direction about a former such that the first major face is convex and the second face is concave. More preferably, the first metal tape is formed from silver alloy and the second metal tape is formed from pure silver. In other embodiments, however, the metal tapes are formed from the same material and the first tape is thicker than the second tape.
Preferably, the first metal tape is flat and has a width not substantially greater than that of the superconducting tapes. In others embodiments, however, the width of the metal tapes is slightly less than the width of the superconducting tapes. In still further embodiments, the first tape is wider than the superconducting tapes. However, in this last embodiment, the protruding edges of the metal tape are preferably bent to form a channel section. It will be appreciated that although this would have structural advantages it would also adversely affect fill factor.
In some embodiments, use is made of silver foil or other compatible material which is wrapped around the stack and extending longitudinally.
Preferably, in embodiments having multiple constituent tapes, those tapes are stacked in a single stack. In other embodiments, however, the tapes are stacked in two or more parallel adjacent stacks. The latter configuration preferably includes one or two full width metal tapes or superconducting tapes to bridge from stack to stack. Those structures with exactly two parallel stacks appear from preliminary experiments to have superior AC loss characteristics.
According to another aspect of the invention there is provided a composite superconducting tape characterised by at least one constituent powder in tube superconducting tape and at least one outer layer of metal tape overlying and bonded to one of the major exposed faces thereof with the proviso that if there are two such metal tapes they differ in strength.
Preferably there are a multiplicity of stacked and bonded constituent superconducting tapes.
In use, the metal tape, or the stronger metal tape if there are two, needs to be on the convex side. A stronger tape may, for example, be a silver alloy tape and the weaker tape pure silver, or the tapes may differ in thickness.
Preferably, such metal tape is flat has a width not substantially greater than that of the superconducting tapes (it might be slightly less). However, if desired a wider metal tape which is, or subsequently becomes, bent to a channel section could be used; this would have structural advantages but would adversely affect fill factor. Similarly the use of silver foil (or other compatible material) wrapped around the stack but extending longitudinally is not excluded, but presently it is considered unnecessary and undesirable, especially as there tends always to be more silver than is useful at the edges of the constituent tapes. When there are multiple constituent tapes they may be stacked in a single stack or in two or more parallel adjacent stacks; the latter will normally require one or two full-width metal tapes (or, if desired, superconducting tapes) to bridge from stack to stack. At least structures with exactly two parallel stacks appear from preliminary experiments to have superior AC loss characteristics.
Unless the context clearly requires otherwise, throughout the description and the claims, the words xe2x80x98comprisexe2x80x99, xe2x80x98comprisingxe2x80x99, and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of xe2x80x9cincluding, but not limited toxe2x80x9d.